Forsterite-rich refractory sand composition

ABSTRACT

A synthetic forsterite-rich refractory sand composition for use as a taphole nozzle refractory sand for a sliding gate system of a steel ladle, the synthetic forsterite-rich sand composition including a mixture, the mixture including: a) a carbon bridging agent, the carbon bridging agent being present in the mixture in a proportion of from 5 to 15 percent by weight of the mixture; and b) a synthetic forsterite-rich sand, the synthetic forsterite-rich sand being free-flowing, having a MgO:SiO 2  weight ratio of at least 1.2, a specific gravity of approximately 3 g/cm 3 , a void volume of less than approximately 50 percent by volume of the synthetic forsterite-rich refractory sand, and a fusion point of at least 1610° C. At least 50 percent by weight of the synthetic forsterite-rich sand is forsterite; and from 8 to 27 percent by weight of the synthetic forsterite-rich sand is maghemite.

This application is a continuation of U.S. application Ser. No.07/839,508 filed Feb. 21, 1992 now U.S. Pat. No. 5,374,593.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to refractory materials produced from tailingsfrom asbestos mines. More specifically, this invention yields suchrefractory materials which are suitable for heat intensive metallurgicalapplications, such as for example in the taphole nozzles of sliding gatesystems in steel mill ladles.

2. Description of the Prior Art

Asbestos deposits normally occur in certain types of silicate rock whichcontain only about 5 to 10% by volume of asbestos fibres. Consequently,separation of the fibres from asbestos ore leaves large quantities ofunwanted tailings which accumulate at or near extraction or processingsites. There is therefore a need to find novel commercial applicationsfor these by-products of asbestos mining and refining operations.

Chrysotile asbestos tailings normally contain a substantial proportionof hydrated magnesium silicates referred to as serpentine. Othercomponents which occur with serpentine rock are brucite Mg(OH)₂ andhematite-magnetite (Fe₂ O₃ -Fe₃ O₄). Deposits of nickel may also occurin the rock. Known potential uses for asbestos tailings include the useof the tailings as a source of nickel extracted by magnetic separationof nickel- and iron-rich magnetic components; the transformation of thetailings into mineral wool by melting with carbon and forming fibers byblowing air through the resulting molten silicate; and the use of thetailings as a source of magnesium and/or magnesium compounds obtained byacid leaching and crystallisation.

It is also known to calcine serpentine in order to produce sinteredangular shaped granules useful as sandblasting or heat accumulatingmaterial, or to produce granular products useful as foundry mold sands.However, conventional calcination methods give rise to a number ofdrawbacks. For example, the conventional process of calcining serpentineresults in the production of a synthetic harzburgite rich in a pyroxeneof the silicate group and commonly referred to as enstatite. Enstatiteis further subdivided into protoenstatite and clinoenstatite dependingon its resulting crystallographic structure. However, these productshave relatively low melting or softening points (less than 1,600° C.),and are therefore not useful as refractory materials for particularlyhigh temperature operations, e.g. in the production of steel whichinvolves exposure to temperature in the region of 1,700° C. for severalhours.

The known methods of serpentine calcination normally involve heating theserpentine to a temperature of about 1,300° C., whereby the followingchemical reactions are said to take place:

(i) dehydration encountered at 600° to 780° C. to form an anhydrousmagnesium silicate: ##STR1## (ii) conversion of the anhydrous magnesiumsilicate into forsterite (Mg₂ SiO₄) and free silica (SiO₂), which startsto occur at approximately 800° to 900° C.: ##STR2## (iii) reaction offorsterite with free silica above 1,000° C. thereby forming enstatite(MgSiO₃): ##STR3##

As previously stated, such enstatite-rich products can find applicationas, for example, foundry mold sands or backing sand mixes. However, theproduct is unsuitable for higher temperature applications in which theproduct is required to have superior refractory properties, for example,in steel or other high temperature smelting operations. Morespecifically, enstatite rich-products are unsuitable as ladle and ladlenozzle sands currently utilized, for example, in steel mills andfoundries because the product is unable to withstand the necessarytemperatures and holding times involved.

Sands currently used in steel mill ladles are mostly silica, zircon andchromite based. Although such products have good granulometry, packingdensity and purity, they are expensive to produce. Moreover, many ofthese products have various degrees of toxicity. Indeed, themanipulation and use of many of such products causes the liberation ofsilicious dusts which are known to engender among workers the seriousmedical condition known as silicosis.

To produce a synthetic olivine from asbestos tailings suitable forextremely high temperature applications, it is theorized that a highproportion of forsterite (Mg₂ SiO₄) should be produced in the finalproduct. This is because of the favorable refractory properties offorsterite compared to enstatite and because the fusion point offorsterite is much higher than that of enstatite. Aitcin, in Am. CeramicSociety Bull., Vol. 61, No.8 (1982) pp. 857 to 860, teaches calciningand sintering non-magnetic portions of asbestos tailings to produce arefractory product. A process is described whereby fine asbestostailings are fired at 1,550° C. However, the resulting refractoryproduct is rich in enstatite. The forsterite content of the refractoryproduct will be a function of the basicity index (MgO:SiO₂) of theasbestos tailings. Hence the forsterite content of the final product islimited by the basicity index of the raw material. For example, atypical fusion product of calcined asbestos tailings consists of 33% byvolume of forsterite and 54% by volume of enstatite, the remainder beingessentially hematite (Fe₂ O₃).

Accordingly, there remains a need to realize an efficient method ofproducing a synthetic olivine composition from asbestos tailings forapplications such as, for example, high temperature ladle nozzle sands,which method will overcome the drawbacks of the prior art.

A first object of the present invention is to provide a process oftreating asbestos tailings in order to convert such tailings into acommercially useful product.

A second object of the invention is to provide a process of producing,from asbestos tailings or other ores of similar composition, syntheticolivine sands having good refractory characteristics at very hightemperatures, such as those encountered during the manufacture andtreatment of steel in steel mill foundries.

Another object of the invention is to produce a forsterite-rich productfrom a serpentine mineral or from a previously-calcined serpentinemineral containing enstatite.

Another object of the invention is to provide a novel refractorymaterial suitable for use at very high temperatures.

Yet another object of this invention is to provide a novel non-toxicsand exhibiting higher refractoriness, greater density and a lowercoefficient of thermal expansion than many conventional high temperaturefoundry sands.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided aprocess of producing refractory material of high forsterite content,which comprises: heating serpentine-containing asbestos tailings, or anenstatite-containing product produced by calcining serpentine-containingasbestos tailings, with magnesium oxide or a magnesium oxide precursorat a temperature of about 1,200° C. or above for a time sufficient toform said forsterite-rich product.

The invention also relates to the novel refractory materials produced bythe process above and to uses of such materials.

It will be noted that the present invention can be used to produce ahigh-melting refractory material either from asbestos tailingscontaining serpentine or from a previously calcined product from suchtailings which contains enstatite and has a relatively low melting point(fusion temperature) of about 1600° C. or lower.

The product of the present invention is a synthetic olivine whichpreferably contains at least 40% by volume of forsterite, morepreferably at least 60% by volume of forsterite, and which optimallyconsists almost entirely of forsterite with very little or no enstatite(preferably less than 10% by volume). The product preferably has amelting (fusion) point of about 1660° C. or higher, and preferably aboveabout 1700° C.

A special advantage of the synthetic olivine of the invention is that itis readily useful in a variety of applications where other refractorysands cannot withstand the required operating temperatures, mechanicalstresses or holding times. The synthetic olivine of this invention isalso free of asbestos fibers, and is essentially non-toxic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of apparatus used in accordancewith this invention;

FIG. 2 is an example of a rotary kiln which can be used to carry out thepresent invention;

FIG. 3 is a graphical representation of analyses and basicity indices ofthe synthetic olivine sands obtained in tests carried out in accordancewith this invention;

FIG. 4 is a graphical representation of the effect of particle sizes onthe fusion points of the synthetic olivine sands obtained in accordancewith this invention;

FIG. 5 is a graphical representation of the effect of the basicityindices on the fusion points of synthetic olivine sands obtained inaccordance with this invention;

FIG. 6 is a schematic cross-section of one possible application of thesynthetic olivine sands of this invention in an industrial setting;

FIG. 7 is a schematic example of a cross-section of the syntheticolivine sands of FIG. 6 during a smelting process.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS

In the present invention, a magnesium compound is used during thecalcination of asbestos mine tailings, or during the heating of asynthetic enstatite-containing product previously produced by calciningsuch tailings, at a temperature of about 1,200° C. or higher, in orderto produce a synthetic olivine compound which is rich in forsterite (Mg₂SiO₄) rather than enstatite (MgSiO₃). Preferably, as noted above, thissynthetic olivine compound will contain more than 60% by volume offorsterite and less than 10% by volume of enstatite.

Without wishing to be bound to any particular theory, it is believedthat, during the heating or calcining step, the presence of the addedmagnesium compound prevents the forsterite resulting from reaction (ii)above from degrading, via chemical reaction with free silica, intoenstatite by reaction (iii). Alternatively, the free silica produced byreaction (ii) may be made to react with the added magnesium compoundinstead of reacting with the forsterite according to reaction (iii).Consequently, a forsterite-rich compound is obtained. In other words, itis believed that a forsterite-rich chemical equilibrium is obtained whenthe reaction of anhydrous magnesium silicate into enstatite is surpassedby the reaction of free silica with magnesium. This is indicated byreaction (iv) shown below:

(iv) reaction of free silica above approximately 1,200° C. withmagnesium oxide: ##STR4##

Alternatively, to achieve similar results, previously calcined asbestostailings, rich in enstatite, can be heated to a temperature of about1200° C. or higher in the presence of suitable amounts of magnesiumoxide or a precursor thereof. It is inferred that some enstatite isreacted back into forsterite, and free silica is reacted with magnesiumto form additional forsterite.

The precise composition of the product obtained in accordance with thepresent invention will normally be dependent on the initial index ofbasicity (MgO:SiO₂ weight ratio, referred to as "Ib") of the batch ofserpentine used, the amount of magnesium oxide or precursor, thecalcining temperatures, and the granulometry of the reactants.

Table 1 below provides typical compositions of tailings from asbestosmines in the Province of Quebec, Canada, and shows the index ofbascisity (Ib) of these materials.

                  TABLE 1                                                         ______________________________________                                        Typical Chemistries of Tailings from Chrysotile Mines                                   Analysis (wt %)                                                     Sample  Ib      MgO    SiO.sub.2                                                                          FeO,Fe.sub.2 O.sub.3                                                                   Al.sub.2 O.sub.3                                                                    LOI*                               ______________________________________                                        Mine 1  0.89    42.8   47.9 4.8      --    13.0                               Mine 2  0.90    35.2   39.2 8.8      1.2   12.1                               Mine 3  1.22    41.4   34.0 9.2      0.4   15.0                               Mine 4  1.25    41.6   33.2 9.1      0.7   13.8                               Example 1                                                                             0.99    37.2   37.7 7.5      0.8   13.7                               below                                                                         Theoretical                                                                           1.00    43.6   43.4 --       --    13.0                               Serpentine                                                                    ______________________________________                                         *Loss on ignition                                                        

The amount of magnesium oxide or precursor required in order to ensure ahigh percentage of forsterite in the product depends on the initial Ibratio, which may range from about 0.8 to 1.2. In theory, 100 g of pureserpentine would produce 10.8 g of silica according to reaction (ii)above, which would require the presence of 14.5 g of magnesium oxide forcomplete reaction with the silica according to reaction (iv). If anexcess of magnesium oxide is provided, this is not harmful because itremains unreacted in the product as periclase which itself has a highfusion temperature. In general, therefore, the required amount of MgO isin the range of 10-25% by weight, and is more preferably at least 15% byweight, based on the amount of asbestos tailings. Another way ofexpressing the amount of MgO required is to state that the Ib of thestarting materials (serpentine and MgO or precursor) should preferablybe in the range of 1.34 to 1.68.

In carrying out the present process, it is not essential to usemagnesium oxide as such since any magnesium compound which yieldsmagnesium oxide under the reaction conditions (i.e. a magnesium oxideprecursor) can be employed, such as for example magnesium hydroxide,magnesium carbonate, magnesium sulfate, or other chemical equivalents ofmagnesium oxide. Naturally, when a magnesium oxide precursor is used,its relative amount should be calculated as magnesium oxide.

At a given MgO:SiO₂ (Ib) ratio, the fusion temperature of the calcinedproduct increases in direct proportion to the fineness of the particlesof the serpentine starting material, possibly because of betterdiffusion of the magnesium oxide into the grains of the serpentine (orforsterite-enstatite, if the reaction is carried out on a previouslycalcined tailings product). The preferred size range of the serpentine(or forsterite-enstatite) is -1.68 mm, in diameter. If necessary, theserpentine starting material may be subjected to a preliminary grindingstep to reduce the particle size as required.

The size of the particles of MgO or precursor is not critical, but theseparticles are desirably of similar size to the serpentine particles inorder to assure good distribution and mixing of the two kinds ofmaterials.

It is preferable to form granular agglomerates of the particles oftailings and the particles of magnesium oxide (or precursor) by mixingthese materials together with water and a binder which remains presentat reasonably high temperature in order to form a paste and then tomaking granules from the paste. The binder prevents disintegration ofthe granules during the calcination step, which could result in loss ofthe magnesium oxide dust from the calciner and an uneven ratio of MgO toSiO₂ in the heated granules. The size and strength of the granules canbe controlled by the amount and dilution ratio of the binder in theaqueous solution. Usually, about 3 to 10% by weight of the bindersolution is required to form suitable granules, and an amount of about5% is normal, depending on the binder chosen. An example of a suitablebinder solution is an aqueous solution of MgSO₄ having the advantage ofbeing a source of MgO.

The mixture of starting materials employed in the present invention isheated at a temperature of at least 1200° C., and preferably at atemperature in the range of 1200° C. to 1500° C., for a length of timenecessary for the desired reactions to reach equilibrium. Ideally, thecalcination temperature is around 1460° C. and the reaction time isusually at least about one hour.

The resulting products, particularly those having Ib ratios over about1.2, have suitable characteristics with respect to size distribution,packing ability, thermal resistance and sintering properties forutilization as ladle sand or for similar refractory uses.

It is especially desirable to mix the refractory product of theinvention with graphite (or other suitable so-called bridging agent)prior to use of the material in refractory applications in particularfor use as a steel ladle taphole nozzle sand. The graphite helps therefractory material to form a low thermal conductivity protectivesintered crust when exposed to high temperatures. The amount of graphiteor other bridging agent is usually in the range of 5 to 15% by weight ofthe resulting mixture, and more usually 5 to 10% by weight.

The major properties required for ladle sands applications arerefractoriness, size distribution, high degree of packing, low thermalexpansion, and ability to form a suitable crust in the upper nozzle. Themost common conventional sands used as nozzle materials are silica,natural olivine and zircon with fusion points ranging from 1,430° C. to1,760° C., 1,540° C. to 1,760° C. and 2,040° C. to 2,220° C.,respectively. The olivines produced by calcining serpentine with MgOaccording to the present invention have fusion points ranging upwardlyfrom 1,610° C. to more than 1,680° C. depending on the nature of thecomposition, and particularly on the MgO:SiO₂ (Ib) ratio. Preferably,the fusion points are above 1,660° C.

The high thermal resistance of the synthetic olivine produced accordingto the present invention in the presence of liquid steel makes itsuitable also for other applications in steelmaking, namely, incastables, plastic and fire bricks, and gunning and ramming materialsresisting to temperatures over 1650° C.

The process of the invention may be carried out continuously in anapparatus of the type shown in schematic form in FIG. 1. In thisapparatus, asbestos tailings (previously ground to a desired particlesize) are fed from a hopper 10 through flow control valve 11 to aconveyor belt 12. The belt then conveys the serpentine particles beneatha stream of magnesium oxide or precursor particles from hopper 14. Theresulting powder mixture is then conveyed beneath a stream of an aqueoussolution of an inorganic binder from tank 15. The resulting paste-likemixture is deposited in a mixer (granulator) 16 and the emerginggranules are fed into the entrance of a downwardly inclined rotary kiln17 in which the required calcination takes place.

The kiln 17 is shown in greater detail in FIG. 2. The granules frommixer 16 are fed to input funnel 21, into feeding zone 22 and proceed toa pre-heating zone 23. Dehydration of the serpentine begins in thepre-heating zone 23. The pre-heated material then proceeds to a staticfurnace calcining and sintering zone 24 where it reaches a desiredtemperature in the range of 1,200° to 1,500° C. The calcined materialthen reaches a cooling zone 25, from which the final product iscollected. The material has a residence time in the kiln 17 of about onehour.

The invention is illustrated by the following Examples but should not beconstrued as limited thereto.

EXAMPLE 1

A test was carried out to determine the effect of the size of rawserpentine particles and of the amount of MgO added to the serpentine onthe fusion point of the calcined products.

Heat treatments of various starting materials were carried out in arotary kiln of the type shown in FIG. 2, subjecting the startingmaterials to an increasing temperature from ambient to 800° C. in afirst step, and then heating was pursued up to 1350° C. in a staticfurnace. The total retention time was 2 hours. In each test, granulationof the serpentine-MgO mixture was employed to prevent kiln loss duringthe calcination stage. The pellet size and strength were controlled bythe amount and dilution ratio of a magnesium sulfate binder in theaqueous solution.

Table 2 shows the composition of the serpentine-MgO mixtures tested, aswell as the chemical analyses of the corresponding calcined products andtheir fusion temperature (T_(f)). Calcining temperatures reachedapproximately 1,350° C. in the static furnace except as otherwiseindicated in Table 2. The temperatures of fusion were measured with aheating microscope calibrated to a maximum reading of 1,680° C. TheMgO:SiO₂ (Ib) ratio was calculated before and after the heat treatment.The ratio values are plotted in the graph shown in FIG. 3 along withcomparative analytical results.

                                      TABLE 2                                     __________________________________________________________________________    Composition of the Feed and Products of Calcination                           SERPEN-   SERPENTINE-MgO                                                      TINE      FEED MIXTURES                                                                             OLIVINE PRODUCTS                                        Test                                                                              Size  %   %       %  %                                                    No. (mm)  MgO*                                                                              SiO.sub.2 *                                                                       Ib  MgO                                                                              SiO.sub.2 *                                                                       Ib T.sub.f                                       __________________________________________________________________________    1   -3.36 41.1                                                                              34.9                                                                              1.18                                                                              48.5                                                                             39.62                                                                             1.24                                                                             1610                                          2   -1.68 41.6                                                                              34.2                                                                              1.22                                                                              48.6                                                                             39.0                                                                              1.25                                                                             1670                                          3   -0.84 41.9                                                                              35.4                                                                              1.18                                                                              46.2                                                                             38.0                                                                              1.22                                                                             1680                                          4   -1.68 39.9                                                                              34.9                                                                              1.14                                                                              48.9                                                                             42.7                                                                              1.15                                                                             1625                                          5   -1.68 44.0                                                                              36.3                                                                              1.21                                                                              49.4                                                                             40.4                                                                              1.22                                                                             1660                                          6   -1.68 46.5                                                                              35./5                                                                             1.31                                                                              51.1                                                                             38.9                                                                              1.31                                                                             >1680**                                       7   -1.68 47.0                                                                              34.6                                                                              1.36                                                                              52.3                                                                             38.3                                                                              1.37                                                                             >1680**                                       8   -1.68 48.0                                                                              33.2                                                                              1.44                                                                              56.9                                                                             36.9                                                                              1.51                                                                             >1680                                         9   -1.68 50.8                                                                              31.3                                                                              1.62                                                                              59.1                                                                             35.0                                                                              1.69                                                                             >1680***                                      10  -1.68 53.6                                                                              29.5                                                                              1.82                                                                              62.6                                                                             31.1                                                                              2.01                                                                             >>1680****                                    11  -1.68 47.6                                                                              32.1                                                                              1.48                                                                              51.0                                                                             34.7                                                                              1.47                                                                             >1680                                         12  -1.68 50.5                                                                              30.3                                                                              1.67                                                                              51.7                                                                             33.0                                                                              1.57                                                                             >1680                                         13  -1.68 53.3                                                                              28.6                                                                              1.86                                                                              59.3                                                                             30.2                                                                              1.96                                                                             >>1680                                        __________________________________________________________________________     *Calculated from the head analyses of the mixed serpentine and magnesium      oxide.                                                                        **These two products started to soften at 1,680° C.                    ***Calcining temperature reaching 1,425° C.                            ****Calcining temperture reaching 1,460° C.                       

FIG. 4 shows the relationship between the serpentine particle size andthe fusion temperature of the resulting products. The graph shows that,at a given MgO:SiO₂ ratio (1.2 in this case), the fusion temperature ofthe calcined product increases with the fineness of the raw serpentine.

Since pure forsterite has an MgO:SiO₂ ratio of 1.34 with a fusiontemperature of 1890° C., it is significant that any compositions havinga higher MgO:SiO₂ ratio have resulting fusion temperatures greater than1680° C. (which was the highest detectable limit of the heatingmicroscope used in this Example).

The effect of the amounts of MgO additions on the fusion temperatures ofthe products is graphically illustrated in FIG. 5.

EXAMPLE 2

This Example shows the testing of forsterite-rich synthetic olivineproduced according to the present invention as ladle sand for a steelslide-gate system.

Three synthetic olivine blended products according to the invention wereprepared and tested for their thermal resistance to liquid steel at hightemperature for extended periods. These products are identified in Table3 with the corresponding basicity index and major mineralogicalcomponents.

                                      TABLE 3                                     __________________________________________________________________________    Blended Products to be Tested as Ladle Sand                                   Blended      MgO:SiO.sub.2                                                                       T.sub.f                                                                            Minerals Identified                                   Product                                                                            Source**                                                                              (Ib)  °C.                                                                         (Approx. Content)                                     __________________________________________________________________________    No. 1                                                                              Tests 2 and 5                                                                         1.22  1665 Principal:                                                                         Forsterite (Mg.sub.2 SiO.sub.4)                                          Major:                                                                             Enstatite (MgSiO.sub.3)                                                  Minor:                                                                             Iron Oxides*                                                                  Magnesioferro peroxene                                                        (Fe, Mg)SiO.sub.3                                No. 2                                                                              Tests 6, 7,                                                                           1.39  >1680                                                                              Principal:                                                                         Forsterite                                            8 and 11           Minor:                                                                             Iron Oxides*                                                                  Magnesioferro peroxene                                                        Enstatite                                                                     Periclase (MgO)                                  No. 3                                                                              Tests 9 and 12                                                                        1.64  >>1680                                                                             Principal:                                                                         Forsterite                                                               Major:                                                                             Periclase (MgO)                                                          Minor:                                                                             Iron Oxides*                                                                  Magnesioferro peroxene                           __________________________________________________________________________     *Maghemite (Y--Fe.sub.2 O.sub.3), hematite (Fe.sub.2 O.sub.3),                magnesioferrite (MgFe.sub.2 O.sub.4)                                          **Test numbers refer to test numbers in Example 1.                            Note: No silica (SiO.sub.2) was identified in the products.              

A proper grain size distribution insures high packing. The sand,therefore, should report on several successive screens in order to avoidtoo many interstices between the grains. Table 4 shows a comparison ofthe size of the forsteritic olivines of this invention with other typesof sands.

                                      TABLE 4                                     __________________________________________________________________________    Comparative Screen Tests of Ladle Sands                                       Size                                                                          Wt  US Screen                                                                           16  20 30 40 50 70 100                                                                              200                                                                              -200                                       %   mm    1.18                                                                              0.84                                                                             0.60                                                                             0.42                                                                             0.30                                                                             0.21                                                                             0.15                                                                             0.08                                                                             0.08                                       __________________________________________________________________________    SILICA SAND                                                                             15  53 31  3 -- -- -- -- --                                         NATURAL   --  -- -- -- 26 35 26 9  4                                          OLIVINE   --  -- --  3 3  4  47 41 1                                          ZIRCON SAND                                                                   PRODUCT NO. 1                                                                           19  20 15 12 9  8  8  6  3                                          PRODUCT NO. 2                                                                           27  23 13 11 8  6  7  3  2                                          PRODUCT NO. 3                                                                           20  18 14 16 11 7  5  5  4                                          __________________________________________________________________________

Other physical characteristics of the olivine sands assure good packingqualities. These characteristics are namely, specific gravity, bulkdensity, and percentage of void volume. Table 5 shows such physicalcharacteristics for each of the synthetic olivines.

                  TABLE 5                                                         ______________________________________                                        Physical Properties of Synthetic Forsterite-rich Sands                                 Specific               Void                                          Product  Gravity    Bulk Density                                                                              Volume %                                      ______________________________________                                        No. 1    2.93       1.46 (91 lb/ft.sup.3)                                                                     50.2                                          No. 2    2.86       1.56 (99 lb/ft.sup.3)                                                                     49.8                                          No. 3    2.82       1.40 (87 lb/ft.sup.3)                                                                     46.8                                          ______________________________________                                    

Both the relatively high specific gravity and a semi-angular shape ofthe olivine products have resulted in good free-flowing material.

Finally, two other factors making the novel sands suitable for tapholenozzle are a low thermal expansion coefficient (1.09×10⁻⁵) and highhardness (5.5 on Mohs scale) for all three olivine products.

In contact with liquid steel, a satisfactory ladle sand must form a lowthermal conductivity layer, known as a crust, at the top of the nozzle.This crust is broken by the static pressure of steel when the slidinggate is opened to tap the furnace. Sintering is obtained either fromnatural occurrence, such as by the presence of feldspar in silica sands,or by making admixtures to this effect, e.g. via additives like graphiteto chromite, silica to zircon, etc. The latter combination produces aglassy bridge at about 1540° C. by the melting of free silica and thedecomposition of zircon into zirconia and silica.

As illustrated in FIG. 6, to assess the suitability of the syntheticolivine products as ladle sands, tests were conducted in an electricalinduction furnace 60 having no tapping hole at the bottom. A refractorymold 61, filled up with an olivine-graphite mixture 62 (containing 10%graphitic carbon), was installed at the bottom of the furnace and wassurrounded with rammed mullite 63 (to avoid side heat transfer). Therefractory mold 61 had a conical section 64 in its upper section and thesand was heaped into a "flower pot" topping 65 in this conical section.The top surface of the sand was protected by a steel plate 66 over whichwas placed a 25 kg steel ingot 67.

The metal was melted and maintained at pre-selected temperatures, i.e.,1600° C., 1650° C. and 1700° C., for periods of time varying from 1 to 2hours as schematically illustrated in FIG. 7.

Since the thickness of a crust 70 formed at the top of the refractorybed 62 depended on the temperature of the liquid steel 72 and thecontact period between the liquid metal and the ladle sand, the twofactors were investigated simultaneously by increasing both thetemperature and the length of time as shown in Table 6.

                  TABLE 6                                                         ______________________________________                                        Conditions of Nozzle Testing with Forsteritic Olivine                                                  Contact                                              Product       Liquid Steel                                                                             Period                                               No.           °C. hr                                                   ______________________________________                                        1             1600       1                                                    2             1650       11/2                                                 3             1700       2                                                    ______________________________________                                    

After the test was completed, the furnace 60 was tilted and the liquidsteel 72 poured. The refractory crust 70 formed at the interphasebetween the olivine-graphite sand 74 and the liquid steel 72 wassufficient to retain the unaltered olivine-graphite sand 74 in the mold,which was pulled out for examination. It was found in every test thatthe crust 70 was thin enough to be easily broken by a small rod. Thisindicated that the sintered layer would break under the pressure of theliquid steel in an industrial sliding-gate system.

EXAMPLE 3

Five tests were carried out in which serpentine tailings were heatedwith magnesia precursors MgCO₃ and MgSO₄. The conditions and results areshown in Tables 7 and 8 below.

                                      TABLE 7                                     __________________________________________________________________________    Granulometry, composition, and basicity index of serpentine, additives        and resulting mixtures                                                                            Additives (weight) %                                                                       Serpentine-MgO--                                       Asbestos     MgO       MgSO.sub.4 Mixtures                          Sample    Tailings     Present             MgO:                               granulo-                                                                             Test                                                                             Analysis (dry) %                                                                           in   MgSO.sub.4                                                                         Analysis (dry) %                                                                        SiO.sub.2                          metry  No.                                                                              MgO                                                                              SiO.sub.2                                                                        Fe.sub.2 O.sub.3                                                                  Ib MgCO.sub.3                                                                         Solution                                                                           MgO                                                                              SiO.sub.2                                                                        Fe.sub.2 O.sub.3                                                                  (Ib)                               __________________________________________________________________________    (-1.68 mm)                                                                           12 39.0                                                                             39.0                                                                             5.9 1.00                                                                             12.2 5.0  46.5                                                                             34.2                                                                             5.2 1.36                                      13 39.0                                                                             39.0                                                                             5.9 1.00                                                                             16.6 5.0  49.1                                                                             32.5                                                                             4.9 1.51                                      14 39.0                                                                             39.0                                                                             5.9 1.00                                                                             21.3 5.0  52.0                                                                             30.7                                                                             4.6 1.69                                      15 39.0                                                                             39.0                                                                             5.9 1.00                                                                             16.6 5.0  49.1                                                                             32.5                                                                             4.9 1.51                                      16 39.0                                                                             39.0                                                                             5.9 1.00                                                                             16.6 5.0  49.1                                                                             32.5                                                                             4.9 1.51                               __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________    Thermal treatment conditions, analysis to olivine product at                  measurement of control temperatures                                           Sample   Calcination                                                                              Olivines Products                                         granu-                                                                              Test                                                                             Temperature (°C.)                                                                 Analysis (%) Temperatures (°C.)                    lometry                                                                             No.                                                                              Stage I                                                                            Stage 2                                                                             MgO                                                                              SiO.sub.2                                                                        FeO.sub.3                                                                         Ib Softening                                                                          Fusion                                  __________________________________________________________________________    (-1.68 m)                                                                           12 800 (1 h)                                                                          1350 (1 h)                                                                          47.7                                                                             37.6                                                                             8.0 1.27                                                                              1680                                                                              >1680                                         13 800 (1 h)                                                                          1350 (1 h)                                                                          48.6                                                                             35.0                                                                             7.6 1.39                                                                             >1680                                                                              >1680                                         14 800 (1 h)                                                                          1350 (1 h)                                                                          50.0                                                                             33.7                                                                             7.5 1.48                                                                             >1680                                                                              >1680                                         15 800 (1 h)                                                                          1405 (1 h)                                                                          49.4                                                                             36.0                                                                             6.8 1.37                                                                             >1680                                                                              >1680                                         16 800 (1 h)                                                                          1460 (1 h)                                                                          50.8                                                                             35.0                                                                             6.0 1.46                                                                             >1680                                                                              >1680                                   __________________________________________________________________________

EXAMPLE 4

Seven samples were produced according to the process of the inventionand analyzed for the content of various mineral phases. The results areshown in Tables 9 and 10 below.

                  TABLE 9                                                         ______________________________________                                        Analyses of the X-ray powder-diffraction patterns                             gives the following uncorrected compositions                                  (expressed as volumetric percentages):                                        Phase       ICDD #   1.    2.   3.  4.   5.  6.  7.                           ______________________________________                                        forsterite  3-189    87    86   96  97   93  91  86                           enstatite   31-634   --    --   11  14    9  51  34                           periclase   4-829    40    59   91  32   26  --  21                           maghemite-C 39-1346  49    72   34  26   16  44  80                           protoenstutite                                                                            3-523    15    14   --  --   --  --  --                           magnesium silicate                                                                        11-273   --    --   --  --   --  41  --                           quartz      5-490    --    --   --  --   --  --   9                           ______________________________________                                    

Correcting these figures for the respective I/I_(c) ratios, andnormalizing, gives the following quantitative compositions:

                                      TABLE 10                                    __________________________________________________________________________    Phase     ICDD #                                                                             1.  2. 3.  4. 5.  6. 7.                                        __________________________________________________________________________    forsterite                                                                              34-189                                                                             65  58 66  75 81  52 54                                        Mg.sub.2 SiO.sub.4                                                            enstatite 31-634                                                                             --  --  5   7 5   19 14                                        (Fe,Mg)SiO.sub.3                                                              periclase 4-829                                                                               8  10 16   7 6   --  4                                        MgO                                                                           maghemite-C                                                                             39-1346                                                                            20  26 13  11 8   14 27                                        Fe.sub.2 O.sub.3                                                              protoenstatite                                                                          3-523                                                                               8   6 --  -- --  -- --                                        MgSiO.sub.3                                                                   magnesium silicate                                                                      11-273                                                                             --  -- --  -- --  15 --                                        MgSiO.sub.3                                                                   quartz    5-490                                                                              --  -- --  -- --  --  1                                        SiO.sub.2                                                                     Total          100 100                                                                              100 100                                                                              100 100                                                                              100                                       Ib ratio       1.27                                                                              1.39                                                                             1.48                                                                              1.48                                                                             1.48                                                                              1.48                                                                             1.48                                      __________________________________________________________________________

Numerous modifications and variations of the present invention may bemade, in light of the above teachings, without departing from the basicspirit of the present invention. Accordingly, it will be appreciated bythose skilled in the art that, within the scope of the appended claims,the invention may be practised otherwise than as specifically describedherein.

What we claim is:
 1. A synthetic forsterite-rich refractory sandcomposition for use as a taphole nozzle refractory sand for a slidinggate system of a steel ladle, said synthetic forsterite-rich sandcomposition comprising a mixture, said mixture including:a) a carbonbridging agent, said carbon bridging agent being present in said mixturein a proportion of from 5 to 15 percent by weight of said mixture; andb) a synthetic forsterite-rich sand, said synthetic forsterite-rich sandbeing free-flowing, having a MgO:SiO₂ weight ratio of at least 1.2, aspecific gravity of approximately 3 g/cm³, a void volume of less thanapproximately 50 percent by volume of said synthetic forsterite-richsand, and a fusion point of at least 1610° C., whereini) at least 50percent by weight of said synthetic forsterite-rich sand is forsterite,and ii) from 8 to 27 percent by weight of said synthetic forsterite-richsand is maghemite.
 2. The synthetic forsterite-rich refractory sandcomposition of claim 1, wherein said MgO:SiO₂ weight ratio is from 1.27to 1.48.
 3. The synthetic forsterite-rich refractory sand composition ofclaim 1, wherein said carbon bridging agent is graphitic carbon.
 4. Thesynthetic forsterite-rich refractory sand composition of claim 1,wherein from 11 to 20 percent by weight of said syntheticforsterite-rich sand is maghemite.